訂閱這個網誌

Follow by Email

Choosing a Saw Blade Based on TPI (Teeth Per Inch)

A common concern among woodworkers involves the selection of blades for their saws. Whether they are working with a table saw, band saw, circular saw, or jig saw, most woodworkers seek the holy grail of saw blades that offer the smoothest, best cuts possible. Woodworkers quickly learn that the number of teeth-per-inch (TPI) on the blade has a big impact on the quality of a cut. The general rule of thumb is "the more TPI, the smoother the cut." The true answer, however, is just a little more complicated than that, as you'll learn once you understand how saw teeth work.

How Saw Teeth Cut

Each tooth on a saw blades is essentially a tiny, sharp chisel that gouges out wood fibers as it speeds through a workpiece. Typically, the more teeth a saw blade has, the smoother the cut will be. This holds true for pretty much any saw you use—table saw, miter saw, circular saw, jigsaw, scroll saw, or band saw. These smooth cuts come at the price of speed, since blades with many teeth achieve their fine finish by cutting slowly. Saw blades with fewer teeth will cut faster and more aggressively, but generally produce somewhat rougher finishes. This is because on a blade with fewer teeth, the spaces (gullets) in-between the teeth are deeper, and the chiseling action of the tooth is more aggressive.

At the same feed speed, a fine-tooth blade with a high TPI count is bringing more teeth to bear on the cut, which means that each tooth is responsible for cutting a smaller amount of wood.

On a rough-cut blade with more teeth, each tooth is chopping out a larger quantity of wood. The obvious consequence is a rougher cut. For this reason, rough-cut blades with a lower TPI count are often reserved for framing carpentry or for cutting workpieces that won't be visible. In fine woodworking using hardwoods, smoothness of cut is more critical, and fine-tooth blades with a high TPI count are more often used.

There are tradeoffs you pay for that smoother cut on a fine-tooth blade. Because the cuts are naturally slower and less aggressive, there is a greater likelihood that the wood will be burned due to friction as the blade feeds through the workpiece. And because the gullets between teeth are very small and don't clear out the sawdust as quickly, there is a greater likelihood of binding when cutting with a fine-tooth blade.

Other Factors Affecting the Quality of a Cut

There are other factors that affect the quality of a cut on a saw blade. The angle of the teeth relative to the plane of the blade is called the set. A blade with a more pronounced set will cut more aggressively, but it will also consume more wood because the kerf (the slot cut by the blade) will be wider. This can be an issue if you're trying to conserve wood on very expensive stock.

The sharpness of a blade can greatly impact the quality of a cut. Dull blades are much more likely to scorch and burn wood because the teeth are not cutting the wood fibers effectively. A dirty blade can also affect the cut. As wood pitch builds up on the surface of the blade, it will be more likely to bind up and scorch the wood. Keep your saw blades clean to ensure smooth cutting.

Manufacturers offer different types of blades for different types of cutting:

Crosscut blades typically have a greater TPI count and smaller teeth, designed to smoothly cut wood perpendicular to the wood grain. The teeth have tips with alternating bevels—teeth with left-facing bevels are alternated with teeth that have right-facing bevels.

Rip-cut blades have a lower TPI count, larger teeth, and deep gullets between teeth. They are designed for cutting in the same direction as the wood grain. The teeth are flat-topped to allow them to sever the wood fibers efficiently.

Combination blades are engineered to both cross-cut and rip-cut with adequate efficiency, although they will do neither job quite as well as blades specifically intended for the purpose.They are a good choice for general framing carpentry or for a casual DIYer who doesn't need great precision.

Recommendations

As a general rule, woodworkers should have several different blades on hand for each saw they own. For instance, on a 10-inch table saw, it's a good idea to have a 40-tooth blade for general purposes, an 80-tooth blade for plywood and veneers, and a specialty ripping blade for making rip cuts.

On a band saw, keep a few different thickness blades depending on the sharpness of the curves being cut. For instance, use a 1/4-inch blade for rather tight curves, a 1/2-inch blade for general band saw cuts and a 3/4-inch blade or 1-inch blade for resawing wood stock. The 1/2-inch, 3/4-inch and 1-inch blades should be 3 TPI (teeth per inch) blades, while the thinner blades should have a greater number of teeth per inch.

Also keep several blade options on hand for your hand power tools, such as circular saws and saber saws. Blades for circular saws come in the same options as are available for table saws—fine-tooth blades for precision cross-cutting, a coarser blade for rip-cutting, and a combination blade for all-purpose framing carpentry. A good blade set for a saber saw or jig saw will include coarse-tooth blades for quick cutting and rip cuts, and fine-tooth blades for smooth crosscuts. Specialty blades are also available for cutting ceramics, plastics, and other materials.